The IAP LS oligonucleotide probes, derived from IAP elements that are expressed in normal mouse lymphocytes, detect restriction fragments which have characteristic strain distribution patters (SDPs) among different strains of mice, making them suitable as multilocus probes for genome mapping. In HindIII digests, LS element probes each react with a limited number of restriction fragments that represent junctions between proviral and flanking DNA. Previously we determined chromosome assignments for 44 IAP LS proviral loci by comparing their SDPs with those of previously typed genetic markers in the C57BL/6 X DBA/2 (BXD) and BALB/c X C57BL/6 (CXB) recombinant inbred (RI) mouse strains. Since over 60% of provirus integration sites are generally shared between inbred strains of mice, mapping in these common stocks provides data for less common strains with genetically determined phenotypes of interest. We have mapped IAP proviral elements that were not polymorphic among the inbred strains in an interspecific backcross between C57BL/6J and Mus spetus in collaboration with Drs. Verity Letts and Wayne Frankel, The Jackson Laboratory. Chromosomal locations have been assigned to 70 LS proviral loci in the C57BL/6J strain. Fourteen proviral loci defined by probes based on IAP elements expressed in plasmacytomas have also been mapped in the interspecific backcross.